The present invention concerns a heating device for an aerodynamic profile. More particularly, the profiles concerned are those for which the aerodynamic shape must not be disturbed by the formation of ice, especially helicopter blades (main rotor or tail rotor), or alternatively aircraft wings.
The problem of icing on such profiles is well known in the aeronautics industry. The shape of the aerodynamic profiles can be modified because of the formation of ice resulting from the undercooling of water droplets contained in the atmosphere which the profile encounters in flight.
This problem is often treated by equipping the profile with a Joule effect heating structure.
Most often, the heating device comprises, incorporated into the aerodynamic profile in the vicinity of a leading edge of the aerodynamic profile, several resistive elements forming a first resistive element set running approximately parallel to the leading edge, each of the said resistive elements being provided at its proximal end with means of connection to an electrical supply, so-called first electrical supply, the device comprising in addition a cyclical supply control to supply, from the aforesaid first electrical supply, at least some of the resistive elements of the first set, one after the other, according to a specified sequence so that the first set of resistive elements forms a de-icing circuit.
The resistive elements of such a heating device, when they are supplied in an intermittent way, dissipate the heat to eliminate the ice which forms regularly on the aerodynamic profile of the leading edge. They have thus a curative action that does not consume very much electrical power and is beneficial in de-icing large surfaces.
However, in the event of failure of the de-icers, the aerodynamic profiles of helicopter blades are no longer protected against ice. Their effectiveness then decreases rapidly with the formation of ice, increasing the risk of accident. In addition, the de-icers have only a curative action, they do not prevent the formation of ice in a preventative manner when the helicopter is placed in very strong icing conditions.
The object of the invention is to propose a heating device that permits freedom from the limitations of the de-icers mentioned above.
To this end, according to the invention, a heating device for an aforementioned aerodynamic profile, is essentially characterised in that it includes a second set of resistive elements incorporated into the aerodynamic profile in the vicinity of the leading edge of the aerodynamic profile and running approximately parallel to the leading edge, each of the said resistive elements being provided at its proximal end with means of connection to a second electrical supply, all the resistive elements of the second set being collectively and selectively supplied by the second electrical supply so that the second set of resistive elements forms an anti-icing circuit, and in that the distal ends of each of the resistive elements of the first and second sets are connected by electrical returns to the first and second electrical supplies, the electrical returns of the first set being independent of the electrical returns of the second set.
In this way, the aerodynamic profile of a blade fitted with such a heating device is provided with means of de-icing and means of anti-icing which provide at the same time a curative and preventative action against the formation of ice. These systems are moreover redundant. In this way, in the event of failure of one of the circuits, the other circuit ensures that the risks of incident are limited.
The heating device, according to the invention, can possibly comprise in addition one or several of the following characteristics:
the resistive elements of the first set and/or the resistive elements of the second set are made in a metallic material;
the resistive elements of the first set and/or the resistive elements of the second set are made of electrically conducting fibres of composite material running approximately parallel to the leading edge of the profile;
the resistive elements of the first set and of the second set are arranged one beside the other in a heating mat that covers the vicinity of the leading edge, at least one resistive element of the second set being located along a lateral edge of the heating mat;
the second set comprises at least one resistive element located along a lateral edge of the heating mat and at least one resistive element located along the opposite lateral edge of the heating mat;
the resistive elements of the first set and the resistive elements of the second set are arranged one beside the other in a heating mat covering the vicinity of the leading edge, at least one element of the second set being located closer to the leading edge than the elements of the first set;
the elements of the first set are arranged one beside the other in a heating mat covering the vicinity of the leading edge, and at least one resistive element of the second set is superimposed on the elements of the first set in the heating mat;
the said second set is composed of a resistive element covering approximately all the width of the heating mat, by superposition of the resistive elements of the first set;
at least one resistive element of the second set is superimposed on a part of the width of the heating mat located at right angles to the leading edge;
at least one resistive element of the second set is placed so as to cover a gap separating two adjacent resistive elements of the first set;
at least some of the returns of the first set of resistive elements are electrically connected to one another, these returns being made of metallic material or made of electrically conducting fibres of composite material; and
the electrical returns connected to one another of the first set are arranged on the aerodynamic profile, approximately in the same way as the resistive elements of the second set.
In this way, according to the number and the relative value of the resistance of the resistive elements of the first and second sets, greater importance can be given to the de-icing or the anti-icing function. This makes the best compromise to be made, in each particular case, between the effectiveness against the formation of ice and the electrical consumption.
When the resistive elements are not highly mechanically loaded, i.e. when the blade is lightly loaded by vibratory fatigue stresses, these resistiveelements can be metallic. The thickness of these elements is then advantageously small, and they are easy to incorporate into the blade.
On the contrary, when the blade is highly loaded by fatigue stresses, the resistive elements of both sets are preferably each made from conducting fibres, typically made of carbon, running parallelly to the leading edge of the profile and this all the more advantageous if the blade is itself of composite material. Achieving this carbon or similar base fibre de-icing/anti-icing has a certain number of advantages:
a major advantage is relative to its mechanical integrity, and therefore to its life span. This mechanical resistance is obtained by the use of composite materials based on carbon fibres, whose static and fatigue resistance properties under the flexing sustained in flight reaches high levels. This characteristic makes it possible to provide de-icers/anti-icers, whose life span is compatible with that of the structure of the blade;
in addition, each resistive element is a layer consisting of multiple carbon filaments, all conductors of current, and thus ensuring a multiple redundancy in the event of rupture of one of the filaments;
the composite/composite bond between the carbon fibre based device and the blade is of very high quality over time;
the carbon fibres of the resistive elements contribute to the rigidity of the blade. As these fibres are very close to the leading edge, their contribution to the drag stiffness (proportional to the square of the distance from the neutral axis) is particularly appreciable. It facilitates the lightening, or even the removing, the ledge of the trailing edge generally responsible for part of the drag forces, which has the disadvantage of increasing the mass of the blade and moving back the position of its centre of gravity;
the carbon fibre based device is heated in a more homogeneous manner over all its surface;
the structure of the composite de-icer is flexible before polymerisation and can therefore be shaped and laid up over wide irregular and changing surfaces. On the contrary, it becomes stiff and solid after polymerisation (handling advantage, ease of covering, risk of defects reduced, quality increased, etc.);
the risk of corrosion is eliminated or at least reduced.